Apparatus for mass spectral analysis



Oct. 23, 1956 w. H. BENNETT 2,763,302

APPARATUS FLOR MASS SPECTRAL. ANALYSIS Filed Aug. 8, 1951 4 Sheets-Sheet1 GLASS CYL llVDff? INYENTOR N WILLARD HBavmsTr Bfim M w I ATTORNEY;

Oct. 23, 1956 Filed Aug. 8. 1951 w. H. BENNETT 2,768,302

APPARATUS FOR "MAS-S SPECTRAL ANALYSIS I 4 Sheets-Sheet 2 IIIII/HI auafJ/r NJW INVENTOR W/LLA/m H. DLN/YETT ATTORNEYS Oct. 23, 1956 Filed Aug.8, 1951 4 Sheets-Sheet 4 INVENTOR W/LLAHD' H. ficlwvf'rr ATTORNEY}APPARATUS FOR MASS SPECTRAL ANALYSIS Willard H. Bennett, Fayetteville,Ark.

Application August 8, 1951, Serial No. 240,965

7 Claims. (CI. 25.0-41.9)

This invention relates to apparatus for mass spectral analysis.

One object of this invention is to avoid errors due to chemicalreactions at the surface of the cathode, of gases being analyzed.

Another object of this invention is to provide a mass spectrometersuited for analysis with either positive or negative ions.

Yet another object of the invention is to provide a method and apparatusfor mass spectral analysis which has only one mass line for each gas ofthe mixture, without the misleading indications of fragmentation ionindications.

Still another object of the invention is to provide a mass spectrometerwith reduced background effects.

Another object of the invention resides in the provision of a method ofmass spectrometry, applicable particularly to hydrocarbons, whereby theresults may be ascertained by a direct reading.

In mass spectrometers of the prior art the gases to be analyzed contactthe heated cathode. The high temperature of this cathode in some caseschanges the composition of the gases being analyzed, whereby the resultsof the analysis show this new composition instead of or in addition tothe desired one. That defect is avoided in the present invention byenclosing the cathode in a cup which is exhausted separately from theremainder of the tube, the cup having a small aperture through whichelectrons are fed into the other parts of the tube.

Background currents may be avoided in the collector system of thisinvention by employing a double grid in front of the collectorelectrode, and connected to the potential of the collector electrode,with an additional grid charged negatively interposed between the doublegrid and the collector element. This adidtional grid will repel allelectrons from the collector.

In the analysis of gases such as hydrocarbons, errors often appear inthe analysis because of fragmentation ions which appear due to the highpotential of the electrons in the ion source. While it has been knownthat these appear, it has not been possible to eliminate them and hencethe final spectrometer readings include unnecessary and misleading masslines. In some instances it has been possible to identify and thenignore the unnecessary mass lines but this has not always been possible.According to the teachings of this disclosure these unnecessary masslines are eliminated by selecting a potential for the ion source belowthe appearance potential of the fragment ions which are first to appearwhen the gas of the heaviest mass to be analyzed is placed in thespectrometer alone. Using that potential for the ion source one may thenproceed essentially as outlined in detail in my prior copendingapplication Radiofrequency Mass Spectrometer, S. N. 196,024, filedNovember 16, 1950, now abandoned, or as outlined in my published articlein volume 21, pages 143-9, of the Journal of Applied Physics (February1950).

'Ynited States Patent "ice Under these circumstances only one mass linewill appear for each different gas in the mixture being analyzed.

This disclosure also illustrates a novel ion source utilizing parallelscreens between which a beam of electrons is passed. The two screens arecharged positively relative to the cathode but are at different positivepotentials. Hence the positive and negative ions formed in the spacebetween the grids (due to the electron bombardment of the gas underanalysis) tend to move toward the two screens, the positive ions movingtoward the screen of lower potential and the negative ions moving towardthe screen of higher potential. The mass of the ions passing throughthese screens can be determined in known ways or by the apparatusdescribed herein.

The apparatus claimed can be embodied in various forms and itsadaptation in one form will now be described in detail.

In the drawings.

Figure 1 is a partly sectional and partly schematic drawing of one formof apparatus.

Figure 2 is a sectional view along line 22 of Figure 1.

Figure 3 is a right-hand end view of Figure 1.

Figure 4 is a schematic diagram of the apparatus when employed forpositive ion analysis.

Figure 5 shows a series of curves used in explaining the invention.

Figure 6 is a partly sectional and partly schematic drawing of anotherform of apparatus.

Figure 7 is a plan view of the several grids used in the tube.

Figure 8 illustrates certain modifications of the system.

Referring to Figures 1 and 2, the tube employs a cathode 50 which hasleads 50a. A metallic cylindrical shield 51 has lead 51a for connectingthe shield 51 to a variable potential approximately the same as that ofthe cathode. Shield 51 has an aperture 51b which is about fourmillimeters in diameter. Cylindrical metal accelerating electrode 52 maybe connected to a high positive potential through lead 52a. Theelectrode 52 has an aperture at its lower end of about two millimetersdiameter. These parts are all mounted in an envelope 53 which has aninlet opening 54 through which gases to be analyzed may be admitted andit also has exhaust outlets 55 and 56 adapted to be connected to exhaustpumps for evacuating the tube. As is apparent, the exhaust outlet 55primarily exhausts the interior of the cylindrical cup electrodes 51 and52 for the following purpose. The gas to be analyzed may have itschemical composition somewhat changed if it strikes the very hotcathode. Therefore, in order to be certain that none of such gas withchanged composition passes into! the main analyzing chamber, the tube iscontinuously being evacuated at exhaust outlet 55 whereby any gaseswhose composition were altered by cathode 5d are promptly removed fromthe envelope 53. The gases to be analyzed are continuously fed intoinlet 54 and are continuously exhausted primarily through outlet opening56.

The voltage on accelerating electrode 52 is held highly positiverelative to cathode 50, say by fifty volts, by applying this muchpotential between the lead 52a and the center tap of the transformerwinding connected to the filament leads 5%. Such connections areillustrated in Figure 4. As the electrons emerge through aperture 521)they pass between the two parallel grids 61 and 63. The averagepotential of these grids will be termed as i for simplicity. Thispotential in some cases is varied. Assume that it is desired to effect avalue of P of 15 volts positive relative to the cathode 53, then onemight apply a positive voltage of l4 volts to grid 61 and a positivevoltage of 16 volts to grid 63. When one is interested in deliveringnegative ions to the analyzer he would make grid 63 with the higherpositive voltage, say 16 volts in contrast to say 14 volts for grid 61.For positive ion analysis the grid 61 has the higher potential. Amagnetic field of not more than about 100 gauss and generally about 20gauss is applied perpendicular to the axis of the tube by means of acurrent in the two coils 21 and 22. As a consequence the electrons fromcathode 53 form a narrow beam, known as a pencil of electrons, as theyemerge from aperture 52b, at a speed of volts. They slow down to a speedof P volts as they enter the region between grids 61 and 63. in orderthat it may be determined how many electrons are being produced, an itemwhich it is often desirable to know in calibrating and using the tube,as will hereinafter appear, a small metallic electrode 62 is connectedin series with a meter I, a battery B, and the cathode 50. The magneticfield is temporarily removed and the voltage on electrode 52 is adjustedto give a maximum electron current, while the voltages on grids 61 and63 are both set at 15 volts initially in order to focus the electronbeam. When the beam is in proper focus the magnetic field is replacedand the voltages on grids 61 and 63 are rendered slightly different.When negative ions are desired for analysis, the grid 61 is lesspositive than grid 63, in other words grid 61 is negative with referenceto grid 63 thus drawing all positive ions to the left and away from theanalyzing grids. The negative ions are then drawn by grid 63 toward theanalyzer to the right. When positive ions are to be analyzed the grid 61is more positive and draws negative ions from the system whereas grid 63is negative relative to grid 61 and draws positive ions into theanalyzer. Adjacent grid 63 there are located grids 64 and 65respectively connected to leads 64a and 65a, and these grids will becharged with potcntials intermediate those of grids 63 and 66. If one isanalyzing with negative ions, as will hereafter appear, these grids willbe charged positively; and if one is analyzing with positive ions thesegrids will be negative. In either case, however, grids 64 and 65 may beomitted but if these grids are omitted some of the sensitivity of theinstrument will be lost.

The mass spectrometer shown in Figure 1 is of the three stage type andis basically described in my article entitled Radiofrequency massspectrometer in the Journal of Applied Physics, vol. 21, No. 2, pages143 to 149, February 1950, and is also described in my prior copendingapplication entitled Radiofrequency Mass Spectrometer, Serial No.196,024, filed November 16, 1950. in view of these prior disclosures thetheory of the three stage spectrometer will not be related in detail, itbeing sufficient to say that it employs three groups of grids with threegrids in each group. Grids 66, 66' and 66" are connected to lead 66a.Grids 67, 67 and 67" are connected to lead 67a and grids 68, 68 and 68are connected to lead 68a. Leads 66a and 68a are connected to a sourceof negative polarity when positive ions are analyzed, and lead 67a isfed with radio frequency potential, as will be explained in greaterdetail later in connection with Figure 4. The three grids of each groupare preferably close together as compared to the spacing between groups.The distance between grids 67 and 67 is preferably different than thatbetween grids 67 and 67 and preferably according to the ratio of 7 to 5.

The grids 69 as shown in Figure 1 are located between the last stage ofthe analyzer 68 and 67", 68a and the screen 7:) over the collector 71.This grid doublet is important in use with positive ions but may beomitted entirely when negative ions are analyzed or may be used asrepelling electrodes by applying a suitable repelling potential Zthereto of negative polarity. This potential is selected as described insaid article and in said prior application and is of a value to repelall background electrons and ions, leaving only those to be analyzed.Grid 76 is connected to cylindrical shield 70S and to lead 70a which fornegative ion analysis may be connected to lead 69a. The collector 71 isconnected to a source of positive potential by its lead 71a, and sinceit is made as positive as any other electrode in the tube, or more so,it will attract the negative ions that have passed grid 70 and repel allof the positive ions that have been accelerated by the other grids.

When positive ions are being analyzed the grid 69 is at the same highpositive potential as the collector 71, thus providing a stoppingpotential Z. The function of the stopping potential Z is explained insaid prior article and in said prior copending application. The grid 70has a negative potential which is as negative as, or preferably morenegative than, any other electrode in the tube to thus prevent negativeions from reaching the electrode 71.

Figure 3 illustrates in more detail, the leads from the tube. Theseveral grids are knitted wire screens, as shown iin Figure 7, mountedin washer-shaped disc holders 720 which holders are supported byhorizontal rods 72 (see Figures 1, 6 and 7). Instead of bringing theleads vertically through the side wall of the envelope they passhorizontally through the horizontal rods and out the right hand end ofthe envelope as shown in Figure 3.

Cylindrical wire screen 73 connects grids 66 and 66 and cylindrical wirescreen 73a connect grids 68 and 68' inside the tube.

It should be noted that the uppermost two horizontal rods 72 are oneither side of the direct path between cathode 50 and the space betweengrids 61 and 63, hence there is nothing to prevent electrons from thecathode from passing to electrode 62.

In Figure 4 a complete schematic diagram for operating the tube as apositive ion mass spectrometer is shown. The amplifier and measuringinstrument 30 is connected between the collector 71 and the ground andmeasures the ions arriving at collector 71.

Variable frequency oscillator 81 provides the variable radio frequencypotentials, and for many types of work a suitable frequency range forthis may be from kilocycles to 10 megacycles. The output of theoscillator 81 is fed through condenser 83 and reactor 84. The dropacross reactor 84 is fed to the middle grids 67, 67' and 67 and also toleads 66a and 6811 through resistors 66b and 68b. Vacuum tube voltmeter82 measures the potential of the oscillator 81 whereby the latter may beadjusted and held constant.

It is possible to focus the electron beam by varying the contact arm 51cthus varying the potential on accelerating electrode 51.

Having described one suitable form of apparatus I will now describe howone may proceed to analyze a mixture of a series of hydrocarbon gases todetermine the formulas of the gases present. One first selects theheaviest gas that might be present in the mixture and introduces asample of it alone into the tube through inlet 54. Then the potential Pis increased from zero by varying rheostat arm 51d (Figure 4) until onlyone mass line may be detected. The mass line is detected by vary ing thevoltage, on lead 60 (which variation is indicated on meter V) through awide range while observing the galvanometer 8% If during such avariation of voltage on lead 60 there is only one voltage value in saidrange at which an output indication is given at instrument 80, it can besaid that there is only one mass line. Then the potential P is increasedstill further until during a sweep of voltages on lead 60 two mass linesappear. This second potential P is the appearance potential at which thefirst fragmentation ion of the gas appears. The potential P is thenreduced gradually until this second mass line barely disappears. Thispotential, which is just below the appearance potential for the lowestfragmentation ion, is then used for analysis of the whole mixture of theseries of gases. With this value of potential P, only one mass line willappear for each gas present in the mixture.

By way of illustration of this process and system, assume that of theseveral gases that may be involved the heaviest one is CxHy. Withreference to Figure 5, we see that as the ionizing voltage (potential P)is increased from zero that We get an ion yield from the gas CmHy itselfupon reaching potential Pl. As the potential is increased to P-2 we getfragmentation ions for the fragment CzH1 1 which appear due tobombardment of the parent gas. As the potential P is increased furtherto a value P3 we get ions due to the fragment CmHy-Z. The differencebetween potentials P1 and P-Z is the potential required to remove onehydrogen atom from a CzHy ion. The difierence between potentials P-1 andP-3 is the potential required to remove two hydrogen atoms from theCIHQ/ ion. The difference between potentials P-1 and P-4 is thepotential required to remove one carbon and three hydrogen atoms fromthe CazHy ion.

It is apparent that if the tube is operated at a potential greater thanP2 that numerous mass lines will appear by reason of the several groupsof fragmentation ions that are produced. These additional mass linesconfuse the final result by showing mass lines for gases not actuallypresent in the original mixture. However, if the potential P is heldbelow the value P2, the only mass lines that will appear will be theproper ones for the gases introduced into the tube, without more.

It is understood that the process of analyzing hydrocarbon gasesdescribed in this application may not only be practiced with theapparatus shown herein but with other apparatus such as the tube of mysaid prior copending applications and that of said article.

An alternate form of tube is shown in Figures 6 and 7. In these figuresall numbers below 51 represent dimensions in millimeters. In this casethe cathode 100 has electrodes therefor 161 and 162 corresponding to theelectrodes 51 and 52 of Figure 1. The inside cup 101 is continuouslyevacuated at outlet 105 while the gas to be analyzed is continuouslyadmitted in inlet 104. The main exhaust outlet 106 is of courseconnected to the same, or a separate, exhaust pump as is outlet 195. Theion source employs three grids 107, 108 and 109 electrically connectedtogether and these are to be held at a potential higher than that of thecathode. Drawing out grid 110 is connected to a high negative potentialto draw ions from the ion source. Following grid 110 there are threegroups of grids of three grids per group, these being numbered 111 to119 inclusive. These grids function as described in said prior articleand in said prior copending application.

Grids 120 and 121 are tied together and to the collector electrode 123and are charged positively to act as a stopping potential Z. The grid122 is charged negatively, in fact more negatively than any other gridof the tube, to thus repel all electrons from the collector.

While in my various forms of mass spectrometers as described herein andin my prior articles and other applications, it is desirable to have thedistances between the middle grids of the groups integral multiples ofthe same unit of distance, this is not necessary if the radio frequencypotentials are applied to the middle grids out of phase. For example, inFigure 8 the middle grids are fed with three phase radio frequencypotential, and therefore the distances between grids are no longerintegral multiples of a unit distance, but are so spaced that the ionsof the desired gases being analyzed pass each grid at zero potential.Hence, any spacing of the middle grids can be used if the phaserelations of the applied potentials are properly selected.

My copending application Serial No. 240,966 filed on even date herewith,entitled Method of and Apparatus for Mass Spectral Analysis withNegative Ions, the disclosure of which is incorporated herein byreference.

In certain circumstances it is desirable to first analyze a gas mixtureby the positive ion method and then with negative ions. A multipoledouble throw switch may be provided to shift the hook-up of the tubefrom the circuit employed for positive ions to that employed fornegative ions.

in some cases, in connection with my invention, the potential P may beabove the potential at which certain fragmentation ions appear providedit is below the potential at which unwanted fragmentation ions appear.For example, in the analysis of certain oils, it is undesirable for anycarbon fragmentation ions to appear but it does little harm for hydrogenfragmentation ions to appear. Hence, in such a case, in initiallysetting potential P, the potential would be raised until a mass linefirst appears which is more than 13 units below the heaviest mass line.At this point, the potential P is then slightly reduced until this extramass line disappears.

As an example, assume a hydrocarbon gas having the followingconstituents: C10H22, CsHzo, Cal-11s, C'IHIS and Cal-I14. We would get amass line at 142, 128, 114, 100, and 86. If the potential P was largeenough to dislodge a few hydrogen ions from these gases, the only effectof that would be to broaden the mass lines or mass line groups, howeverthey would still peak near the position for the parent unfragmented ionsand would be readily distinguishable from the peaks corresponding tocomponents with different numbers of carbon atoms in the molecule.Should the potential P be increased until carbon fragmentation ionsappeared, a large number of mass lines might appear which would confusethe results. Hence, this disclosure contemplates operation at apotential below the point where carbon fragmentation ions appear.

i claim to have invention:

1. In a mass spectrometer, means for selectively ionizing a gas byelectron bombardment including, electron emission means, electronaccelerating means cooperating therewith, envelope means for maintaininggas to be analyzed in the path of the accelerated electrons, and meansfor selectively controlling the electron accelerating potential; andmass specrum analysis means responsive to the ions developed by thebombardment of the gas by the accelerated electrons.

2. In a mass spectrometer, means for selectively ionizing a gas byelectron bombardment including, electron emission means, electronaccelerating means cooperating therewith, envelope means for maintaininggas to be analyzed in the path of the accelerated electrons, and meansfor selectively controlling the electron accelerating potential over arange that includes a potential which is below the one at whichfragmentation ions are produced; and mass spectrum analysis meansresponsive to the ions developed by the bombardment of the gas by theaccelerated electrons.

3. In a mass spectrometer, means for selectively ionizing a gas byelectron bombardment including, electron emission means, electronaccelerating means cooperating therewith, envelope means for maintaininggas to be analyzed in the path of the accelerated electrons, and meansfor selectively controlling the electron accelerating potential over arange that includes a potential which is below the one at which any gasfragmentation ions are produced; and mass spectrum analysis meansresponsive to the ions developed by the bombardment of the gas by theaccelerated electrons.

4. A mass spectrometer as defined in claim 2 in which said means forselectively ionizing a gas includes an ion source; said mass spectrumanalysis means including all of the following: ion segregating andmeasuring means for indicating the flow of ions that exceed apredetermined energy, a plurality of ion energy increasing means locatedbetween the ion source and the ion segregating and measuring means forsuccessively increasing the energy of ions of a predetermined massduring their 7 journey from the ion source to the ion segregating andmeasuring means, and means including a source of radio frequency energyfor energizing the ion energy increasing means to increase the energy ofions of a predetermined mass more than of other masses.

5. In a mass spectrometer, an envelope having an inlet for receiving thegas to be analyzed, a cathode in said envelope, electrode means in theenvelope and spaced from the cathode, a source of direct currentpotential connected between the cathode and said electrode means andincluding means for varying the potential applied between the cathodeand the electrode means over a range wide enough to include potentialsat which first fragmentation ions of hydrocarbon gases will not appear,and mass spectrum analysis means in the envelope responsive to the ionsdeveloped by the bombardment of the gas in the envelope by electronsfrom the cathode.

6. In a radio frequency mass spectrometer, an ion source, means forselecting and indicating the flow of ions that exceed a predeterminedenergy, first and second devices respectively energized by radiofrequency potentials of the same frequency but of different phase forsuccessively increasing the energy of the ions of a particular mass ontheir journey from the ion source to the first-named means more thanthey increase the energy of ions of other masses, and radio frequencygenerating means connected to said devices for supplying said radiofrequency potentials thereto.

7. A radio frequency mass spectrometer comprising an evacuated envelopehaving an ion source including a cathode electrode, at least threegroups of electrodes with each group having three electrodes, thespacing between groups being large as compared to the spacing betweenelectrodes of the groups, means for applying direct current potentialsto the outer two electrodes of each group, radio frequency potentialgenerating means for applying radio frequency potentials of differingphases to the several middle electrodes, and means for deflecting awayfrom the collector electrode all ions that have passed said electrodeswhich failed to attain a predetermined energy and for measuring the flowof the ions that passed said electrodes in excess of said predeterminedenergy, the spacings of the middle electrodes and the phase relations ofthe potentials applied thereto being so related that the energy of ionsof predetermined mass increases as it passes each group of electrodes.

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Radio Frequency Mass Spectrometer, by Bennett published in Journal ofApplied Physics, vol. 21, dated February 1950, pages 143-149.

Recent Advances in the Production of Heavy High Speed Ions Without theUse of High Voltages, by Sloan et al., published in Physical Review,vol. 46 No. 7 (second series), dated October 1, 1934, pages 539-542.

A Mass Spectrum Analysis of the Products of Ionization by ElectronImpact in Nitrogen Acetylene, Nitric Oxide, Cyanogen and CarbonMonoxide, by Tate et al., published in Physical Review, vol. 48, datedSeptember 15, 1935, pages 525-531.

Gas Analysis with the Mass Spectrometer, by John Hipple, published inJournal of Applied Physics, vol. 13, September 1942, pages 551-559.

Ionization and Dissociation by Electron Impact: The Methyl and EthylRadicals, by Hipple et al., published in Physical Review, vol. 63, Nos.3 and 4, February 1 and 15, 1943, pages 121-123.

